Dielectric composition having anticorona properties

ABSTRACT

A composition suitable for use as a dielectric comprises a base polymer consisting of from 70 to 95% by weight of an amorphous saturated copolymer of ethylene with propylene or butene, the said copolymer containing from 10 to 80% mol. of ethylene and having a Mooney viscosity (ML 212 DEG  F.) of 20 to 80, the remaining 30 to 5% by weight of base polymer consisting of polyethylene, up to 120 parts by weight per 100 parts of the base polymer of an anhydrous oxide of zinc, iron, aluminium, silicon or titanium; an oxyhydrate of aluminium or silicon; a carbonate of calcium, zinc, barium, strontium or magnesium; or a silicate with zinc, calcium, magnesium or aluminium base as a filler, and an organic peroxide and sulphur as vulcanizing agents.ALSO:A band of textile material (e.g. cotton, flax, hemp, glass, regenerated cellulose, polyester) is coated on one or both sides with a composition comprising a base polymer consisting of from 70-95% by weight of an amorphous, unsaturated copolymer of ethylene with propylene or butene, the said copolymer containing from 10-80% mol of ethylene and having a Mooney viscosity (ML 212  DEG F.) of from 20-80, the remaining 30-5% by weight of base polymer consisting of polyethylene, up to 120 parts by weight per 100 parts by weight of the base polymer of an anhydrous oxide of zinc, iron, aluminium, silicon or titanium; an oxy-hydrate of aluminium or silicon; a carbonate of calcium, zinc, barium, strontium or magnesium; or a silicate with zinc, calcium, magnesium or aluminium base as a filler, and an organic peroxide and sulphur as vulcanizing agents.

United States Patent Ofitice Patented Jan. 11, 1966 3,228,883 DIELECTRIC COMPOSETION HAVING ANTI- CORONA PROPERTIES Enzo Di Giulio, Ferrara, and Gerolamo Marchesini, Milan, Italy, assignors to Montecatini Societa Generale per llndustria Mineraria e Chimica and Pirelli Societa per Azioni, both of Milan, ltaly No Drawing. Filed July 27, 1962, Ser. No. 213,026 Claims priority, application ltaly, Aug. 28, 1961, 15,882/61 2 Claims. (Cl. 252--63.2)

This invention relates to electrically insulating materials and compositions.

Electric cables which are insulated by natural and/or synthetic rubber are known. However, the use of such cables is limited to a range of relatively low voltages not exceeding about 2,000 volts owing to their inadequate resistance against ozone and ionization effects.

These two basic drawbacks, which are particularly felt within the range of mean voltages (3,000 to 35,000 volts) and high voltages (over 35,000) volts are due to the nature of the insulating material itself as well as to unavoidable manufacturing defects.

As far as the nature of the material is concerned, this material should obviously be ozone-proof and satisfactorily meet well known mechanical qualifying tests. It was not possible heretofore to develop a material meeting both requirements.

As far as manufacturing defects are concerned, the latter are of greater consequence as the cable is of more complex structure, this involving the number and arrangement of its various constituents (leads, screens, insulating layers, possible reinforcements, etc.). They result in local lack of adherence between the insulation and adjacent screen, apart from unavoidable air bubbles trapped within the insulation. Such discontinuities and air occlusions are to be found in other electric units, such as bushing insulators, transformers and the like.

In accordance with the well known law of distribution of potentials in composite dielectrics formed by two series layers differeing in dielectric constant, the electric potential tends to localize prevailingly on the layer of a dielectric constant. Considering that dielectrics currently employed are of a dielectric constant which is three to four times the dielectric constant of air, the reason will easily be understood why the difference in potential tends to establish across each air bubble or discontinuity due to lack of adherence, till it reaches the critical sill value beyond which air starts ionizing.

In the case of alternating current the ionized particles are subjected to a persistent vibrational movement which is detrimental under various aspects to the dielectric, more particularly:

(a) Local heating up to a temperature at which the dielectric decomposes by purely thermal effect;

(b) Breakage of the long free radical polymer molecules, and consequent deep depolymerization by effect of the impact of ions accelerated by the electric field;

(c) Oxidation of the polymer by the action of ozone and atomic oxygen which are formed by efiect of ionic bombardment. If ionization occurs at a region where the cable is bent, deep cracks are liable to occur at the section of the dielectric subjected to traction and obviously lead to burning of the cable owing to inadequate dielectric rigidity.

An object of this invention is to provide an elastomer dielectric composition and dielectric material comprising said composition, which is free from the above mentioned drawbacks and can be successfully employed within the mean and high voltage range. A further object of this invention is to provide an elastomer dielectric composition which is subject to negligible losses in the dielectric and is unaffected by moisture. A still further object of this invention is to reconcile the above mentioned objects with requirements of a mechanical and technological nature, or the desired easy workability, extrusion, injection, etc. of the dielectric composition and the desired flexibility and strength against mechanical stresses of the dielectric in use in connection with cables, insulating packings and other uses.

In order to meet the above and further purposes this invention starts from amorphous saturated copolymers of ethylene with an alpha-olefin, the latter preferably comprising propylene or butene.

Applicants previously proposed the use of the said copoly-mers for insulating electric cables, more particularly in admixture with mineral fillers and metal soaps. However, it is significant in connection with a purpose of this invention that the dielectrics thereby obtained withstand relatively moderate voltages only owing to the high losses in the dielectric and excessive increase in said losses when the dielectric operates in a moist medium. Cables insulated by this dielectric will yield satisfactory results within the low voltage range (up to about 2,000 volts) but are unsuitable within the mean voltage range or voltages ranging between 3,000 volts and 35,000 volts.

The improved elastomer composition comprises a base polymer content comprising 70-95% by weight of an amorphous saturated copolymer of ethylene with an alpha-olefin, the remaining 305% comprising a homopolymer or copolymer which is co-vulcanizable with the said amorphous saturated copolymer, and a non-hygroscopic mineral filler by a proportion up to 120 parts by weight to parts of the said base polymer content, the said composition containing a peroxide and sulphur as vulcanizing agents.

The mineral filler proportion varies depending upon the nature and grain size of the filler and is generally not lower than 5% by weight of the polymer content. Suitable fillers for the purposes of this invention are, either alone or jointly, anhydrous oxides of dine, iron, aluminum, silicon and titanium; oxyhydrates of aluminum and silicon, carbonates of calcium, zinc, barium, strontium and magnesium, lithopone and barium sulphate; silicates essentially of zinc, calcium, magnesium and aluminum.

The ethylene copolymer shall contain 10 up to 18 moles percent of ethylene; however, it is important for its Mooney viscosity (ML-212 F.) to range between about 20 and 80. The technique for preparing such a copolymer is well known to experts and need not be further dealt with here.

The polymer or copolymer co-vulcanizable with the ethylene copolymer is selected from the class consisting of polyethylene, natural rubber, butadiene-styrene copolymers, synthetic polybutadiene and polyisopropene. The term co-vulcanizable should be understood to mean that the co-vulcanizable polymer or copolymer shall be sensitive in respect of vulcanizing agents (sulphur and peroxide) by substantially the same extent as the ethylenealpha olefin copolymer, so that both shall vulcanize within the same period and under the same vulcanizing conditions. The criteria for selection are already well known in the art and the experts shall have merely to choose from the above indicated class.

Obviously, in addition to the above-mentioned components the elastomer dielectric composition according to this invention will contain antioxidants, lubricants, plasticizers, and carbon black, depending upon requirements. As far as anti-oxidants are concerned, it should be considered that the composition is vulcanized by means of sulphur and peroxides (such as d-icumylperoxide or tertia-rybutylperoxide), so that antioxidants should be compatible with the latter. Anti-oxidants tested in connection with this invention, which afford reliable results, are generally those belonging to the alkyl-phenol class, as well as polymerized dihydroquinoline; of course, various other antioxidants may be suitable for the purpose.

The nature of the lubricant and plasticizers is not critical. In addition to generally known ones, polymers and co-vulcanizable copolymers of low molecular weight (viscosity 500 to 30,000 poises at 20 C.) more particularly lithiumand sodium-polybutadienes of the said viscosity can be satisfactorily employed.

It is further possible to employ metal soap.

The carbon black acts as so-called reinforcement and can be used for the purpose of this invention when the modulus of elasticity and/ or abrasion strength of the dielectric should be improved. Results obtained by applicants with the various types of currently employed carbon black were extremely surprising and disclose the critical character of the additions of this component. For instance, excellent results were obtained with MT (Medium Thermal) carbon black; FT and MPC carbon blacks gave results which may be termed good while carbon blacks known by the International designations EPC, HAF, FEF and SRF led to ready burning of the dielectric at the voltages applied during the tests.

Example A mix of the following composition with the following percentages by weight was prepared:

After vulcanization during two hours at 145 C. the following results were obtained:

Mechanical properties:

Tensile stress kg./sq. mm 0.825 Elongation at break percent 785 ASTM permanent deformation do 17 Retention after ageing in an air flask during 20 h. at 127 C.:

Tensile strength percent 99 Elongation at break 'do 95 Water absorption, 7 g. at

70 C. mg./sq. in. 9 Electric properties:

Dielectric rigidity kv./mm 47 Insulating constant "ohm/km" 30,000 Dielectric constant 2.5 Loss in the dielectric: At 23 C.

40 v./mil 0.005 80 v./mil 0.018 At 100 C.

40 v./mil 0.040 80 v./mil 0.045

From the above mix a cable was manufactured for kv. with a lead of 100 sq. mm. cross sectional area and an insulator thickness of 11 mm. by extrusion and vulcanization during two hours at 145 C. A manufacturing defect was artificially obtained in this cable in the form of an air occlusion, so that the ionization voltage was 8 kv. The cable withstood during a number of months an alternating voltage (50 Hz.) of 60 kv. without exhibiting either burns or variations in its electric properties.

Similar mixes were prepared by replacing polyethylene by any of the following polymers or copolymers: natural rubber, polybutadiene, butadiene-styrene coploymer, the results being practically the same as indicated above.

Whereas similar cables insulated by conventional materials (natural rubber, butyl rubber, polyethylene, butadiene-styrene and the like) submitted to similar testing conditions burnt during a period ranging between a few hours and a few minutes. Only plasticized polyvinyl chloride exhibited a satisfactory strength against break down comparable with dielectrics according to this invention, whereas its losses in the dielectric were considerably high and unacceptable for many uses; moreover, the high mechanical rigidity of the dielectric precludes the use of such material in the flexible cable field.

Considering the circumstance that cables for mean volt ages were insulated heretofore by means of oil-impregnated paper, as well as the structural complexity, manufacturing cost and weight of such cables, the results obtained by this invention are doubtless surprising. The cables manufactured according to this invention are of light weight, easy to lay and reliable in operation, do not require any protective sheathing (as is essential with paper-insulated cables) against moisture and chemical aggressives.

Of course, the use of the invention is not limited to the cable field only. Mixes according to this invention are in fact easily calendered and injection molded even to complex shapes; it was possible to manufacture therefrom electric couplings and cable terminals for external use also and insulating bushing packings withstanding over one hundred kv. without corona effects or breakdown of the dielectric occurring. Mean-voltage experimental transformers were further manufactured, having their core and windings embedded by injection molding in the dielectric according to this invention, the latter forming at the same time the bushing insulators. Such transformers have been operating for months under load without any objection. Of course, a number of further uses are possible, wherever a low or mean voltage lead and an insulating dielectric for such lead are provided.

A very important consequence of the anti-corona properties of the improved compositions concerns any reinforcement provided in the insulating layer and insulating members made from such compositions.

Compositions heretofore employed are hardly suitable in connection with textile or wire reinforcements within the insulator, both on account of the fact that the provision of such reinforcements highly increases the probability of air occlusions in the insulator and the reinforcement material such as cotton, is a cause for corona discharges, finally on account of the fact that a large number of synthetic fibers, which are per se highly insulating, are degraded by a greater or smaller extent during curing of the dielectric.

The high anti-corona effect of the improved compositions makes possible incorporation of reinforcement of natural fibers (such as cotton, flax, hemp) or synthetic fibers (glass, regenerated cellulose, polyester) without any ionization risk and affording at the same time an improved natural resistance of the said fibers under curing conditions. The scope of the invention therefore extends to insulating materials in which a dielectric composition of the above described type is supported for instance on a: band of textile material, or coats both faces of the band, the resulting composite band being useful for forming; insulating layers on an electric conductor.

What we claim is:

1. A dielectric composition having anti-corona properties, consisting essentially of a base-polymer material consisting of 7095% by weight of an amorphous saturated copolymer of ethylene with a member selected from the group consisting of propylene and butene, said copolymer consisting essentially of 10 to mol percent ethylene and having a Mooney viscosity (ML-212 F.) between 20 and 80, the remaining 305% consisting of polyethylene covulcanizable With said copolymer, up to 120 parts by weight based on 100 parts by Weight of said material, of a filler selected from the class consisting of anhydrous oxides of zinc, iron, aluminum, silicon and titanium, oxyhydrates of aluminium and silicon, carbonates of calcium, zinc, barium, strontium and magnesium, lithopone, barium sulfate, and silicates of zinc, calcium, magnesium and aluminum; and an organic peroxide and sulphur as vulcanizing agents, said organic peroxide selected from the class consisting of dicumyl peroxide and tertiary-butyl peroxide.

2. An electric component consisting essentially of a conductor and a dielectric for insulating said conductor, said dielectric being an elastomeric composition consisting essentially of a base-polymer material consisting of 70-95% by Weight of an amorphous saturated copolymer of ethylene with a member selected from the group consisting of propylene and butene, said copolymer containing to 80 mol percent ethylene and having a Mooney viscosity (ML-212 F.) between and 80, the remaining 5% consisting of polyethylene covulcanizable with said copolymer, up to 120 parts by weight based on 100 parts by weight of said material, of a filler selected from the class consisting of anhydrous oxides of zinc, iron, aluminum, silicon and titanium, oxyhydrates of aluminum and silicon, carbonates of calcium, zinc, barium, strontium and magnesium, lithopone, barium sulfate, and silicates of zinc, calcium, magnesium and aluminum; and an organic peroxide and sulphur as vulcanizing agents, said organic peroxide selected from the class consisting of dicumyl peroxide and tertiary-butyl peroxide.

References Cited by the Examiner UNITED STATES PATENTS 2,832,748 4/1958 Satford et a1. 260-455 2,888,424 5/1959 Precopio et al. 26094.9 XR 2,912,410 11/1959 Cole 2 45.5 2,921,872 1/1960 McGlamery 26094.9 XR 2,993,876 7/1961 McGlamery 260-949 XR OTHER REFERENCES Vanderbilt Rubber Handbook, Vanderbilt Co. (1958), pp. 158-9.

JULIUS GREENWALD, Primary Examiner.

ALBERT T. MEYERS, Examiner. 

1. A DIELECTRIC COMPOSITION HAVING ANTI-CORONA PROPERTIES, CONSISTING ESSENTIALLY OF A BASE-POLYMER MATERIAL CONSISTING OF 70-95% BY WEIGHT OF AN AMORPHOUS SATURATED COPOLYMER OF ETHYLENE WITH A MEMBER SELECTED FROM THE GROUP CONSISTING OF PROPYLENE AND BUTENE, SAID COPOLYMER CONSISTING ESSENTIALLY OF 10 TO 80 MOL PERCENT ETHYLENE AND HAVING A MOONEY VISCOSITY (ML-212*F.) BETWEEN 20 AND 80, THE REMAINING 30-5% CONSISTING OF POLYETHYLENE CONVULCANIZABLE WITH SAID COPOLYMER, UP TO 120 PARTS BY WEIGHT BASED ON 100 PARTS BY WEIGHT OF SAID MATERIAL, OF A FILLER SELECTED FROM THE CLASS CONSISTING OF ANHYDROUS OXIDES OF ZINC, IRON, ALUMINUM, SILICON AND TITANIUM, OXYHYDRATES OF ALUMINUM AND SILICON, CARBONATES OF CALCIUM, ZINC, BARIUM, STROTIUM AND MAGNESIUM, LITHOPONE, BARIUM SULFATE, AND SILICATES OF ZINC, CALCIUM, MAGNESIUM AND ALUMINUM; AND AN ORGNAIC PEROXIDE AND SULPHUR AS VULCANIZING AGENTS, SAID ORGANIC PEROXIDE SELECTED FROM THE CLASS CONSISTING OF DICUMYL PEROXIDE AND TERTIARY-BUTYL PEROXIDE. 